[flexiglass] Stop consuming window inset in WindowRootView when flexiglass is on

WindowRootView no longer applies left and right margins according to window insets when flexiglass is enabled - it will be left up to individual scenes to consume the insets appropriately, using WindowInsets provided by Compose and ScreenDecorProvider.

Bug: 324113487
Test: manually verified insets are no longer applied in landscape shade with flexiglass on
Test: manually verified insets are still applied in landscape shade with flexiglass off
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: I90a3834cc5e75efc757d7ba75a48f80040b2ebe7